Type I natural ceramides exist in human skin stratum corneum in a high proportion. The proportion of type I natural ceramides, however, is found to significantly decrease in the stratum corneum of people suffering from, particularly, atopic dermatitis. Type I natural ceramides are a very vital component in maintaining skin homeostasis, and they have been drawing attention as a moisturizer in recent years.
A generally known type I natural ceramide is a compound having a structure represented by the following formula (4):

in which sphingosine is linked to an ω-acyloxy long-chain carboxylic acid via an amide bond (Patent Document 1). Also known in the art is a ceramide derivative composed of phytosphingosine and an ω-acyloxy long-chain carboxylic acid, as represented by the following formula (5).

This derivative has been regarded as a type I natural ceramide (Patent Documents 1 and 2).
Patent Document 1: JP 3782101 B
Patent Document 2: JP H08-502961 T